1. Field of the Invention
The invention relates to hybrid electric vehicle powertrains having all-wheel drive and four-wheel drive capabilities.
2. Background Art
Known transmission designs for hybrid electric vehicles typically include torque transmitting gearing to establish torque delivery from dual power sources (i.e., an engine and an electric motor) to vehicle traction wheels. The torque transmitting gearing may have a fixed ratio or more than one ratio depending upon its configuration. The motor, which is electrically coupled to a generator, can be connected to the traction wheels through a torque flow path in parallel disposition with respect to a torque flow path from the engine. A planetary gear unit may be used to establish the torque flow paths for the engine and the motor. A battery sub-system acts as an energy storage system for the generator and the motor. A hybrid electric vehicle powertrain of this type is disclosed in co-pending patent application Ser. No. 10/605,313, filed Sep. 22, 2003, which is assigned to the assignee of the present invention. Reference may be made to that co-pending application to supplement the present disclosure. That co-pending application is assigned to the assignee of the present invention.
Because of the speed ratio of the gearing in a powertrain of the type disclosed in the co-pending application, the generator speed can be varied to achieve control of the engine speed so that the engine may operate at its most efficient brake specific fuel consumption point. The gearing divides engine power output into a mechanical power flow path and an electrical power flow path to a power output driveshaft for an axle assembly for a pair of traction wheels. The operating characteristic of the powertrain of the co-pending patent application has a functional similarity to the characteristic of a conventional continuously variable transmission in an automotive vehicle powertrain that does not rely upon battery power.
It is possible for a powertrain of the type shown in the co-pending application to use motor and battery power independently of the engine to power the vehicle.
Unlike the powertrain disclosed in the co-pending application, which is characterized as a two-wheel drive powertrain, an all-wheel drive hybrid electric vehicle powertrain is disclosed in co-pending patent application Ser. No. 10/747,429, filed Dec. 29, 2003, which also is owned by the assignee of the present invention. The invention disclosed in that co-pending patent application includes a traction motor disposed on a front wheel axis of the vehicle, thereby providing driving torque to the vehicle front wheels as engine power and generator power are distributed through divided power flow paths to the rear traction wheels.
U.S. Pat. No. 6,648,785 shows a hybrid electric vehicle powertrain configuration with four-wheel drive capability in which a torque transfer case distributes power from a multiple ratio power transmission to both the front and rear axles for the vehicle traction wheels. It includes an electric motor/generator as well as an internal combustion engine. Unlike the powertrains discussed in the preceding paragraphs, wherein an electric motor/generator and the engine can be used either separately as a power source or in combination for delivering power through parallel power flow paths to the traction wheels, the invention of the '785 patent includes a transfer case with two power output elements that are connected separately to a secondary driveshaft for the front traction wheels and the primary driveshaft for the rear traction wheels. Planetary gearing is used to augment the torque distributed to the secondary driveshaft from the electric motor. Engine power can be distributed to either or both of the driveshafts.